It can be used as an lvalue in an assignment in C
"""
6.5.2.5-5
The  value  of  the  compound  literal  is  that  of  an  unnamed  object  initialized  by  the initializer  list.  If  the  compound  literal  occurs  outside  the  body  of  a  function,  the  object has  static  storage  duration;  otherwise,  it  has  automatic  storage  duration  associated  with the enclosing block.
"""

Reduced test:
---
struct S { int a; int b; };
void test()
{
    (struct S) { 1, 2 } = (struct S) { 3, 4 };
}